Microstructure elements and process for the production thereof

ABSTRACT

A microstructured mold is produced from a solid body (metal, ceramic, glass, stone or monocrystalline material) by precision mechanical machining, additive or subtractive structuring. The mold insert is filled and covered by a flowable material and the solidified material is separated from the mold insert giving a microstructure element. The microstructure elements produced by the process have good material properties and can be prepared in a broad range of shapes and structures.

This is a Division of application Ser. No. 08/208,296 filed on Mar. 10,1994, now U.S. Pat. No. 5,501,784.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to microstructure elements and to a process forthe production of microstructure elements from a metal, plastic orsintered material.

2. Discussion of the Background

Microstructure elements have dimensions in the micron range; they arerequired, in particular, in precision engineering, micromechanics,microoptics and microelectronics, for example as sensor elements,actuator elements or components in fluid or electronic systems. They aregenerally employed where properties such as low bulk, low weight andinexpensive manufacture are required. The invention has the object ofproviding microstructure elements of this type in an economic manner.

It is known that microstructure elements can be produced from plastic,metal or ceramic by the LIGA process by lithography, electroforming andcasting. See Kernforschungs-zentrum Karlsruhe, Report 3995 (1985).Plastic microstructure elements are obtained inexpensively and in largenumbers by multiple casting from a metallic mold insert by reactioninjection molding or injection molding.

The primary structure is obtained by imagewise irradiation of aradiation-sensitive plastic with X-rays or synchrotron radiation,followed by dissolution of the irradiated (or unirradiated) areas of theplastic. The mold insert is produced by electrodeposition of metal inthe pre-dissolved areas of the primary structure. The structure of themold insert is complementary to the primary structure. In spite of allthe advantages offered by the LIGA process, such as, for example,substantial geometric freedom in a plane and the wide variety ofmaterials that can be employed, simpler processes are desired in manycases.

It is furthermore known that crystalline materials can be structured byanisotropic etching (Proceedings of the IEEE, Vol. 70 (1982), No. 5, andIEEE Trans. Electron. Devices (ED-25 (1978), No. 10, 1178-1185)). Theresultant microstructure elements can only rarely be used directly,since the etched material does not satisfy certain requirements—forexample adequate breaking strength.

SUMMARY OF THE INVENTION

One object of the invention, therefore, is to produce microstructureelements from a metal, plastic or sintered material by casting from amold insert less expensively and more rapidly and to extend the range ofstructural shapes which can be achieved at acceptable expense.

This object is achieved according to the invention by

(a) producing a microstructured mold insert (primary structure) havingat least one cavity open on one surface thereof from a solid body byprecision mechanical machining, additive structuring or subtractivestructuring;

(b) filling and covering the primary structure with a flowable material;

(c) solidifying the flowable material; and

(d) separating the solidified flowable material from the microstructuredmold insert, giving the metal, plastic or sintered materialmicrostructure element, whose structure is complementary to the primarystructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1 b and 1 c are angled views of three embodiments of theprimary structure or microstructured mold insert of the process of theinvention.

FIGS. 2a, 2 b and 2 c show cross sections of the primary structures ofFIGS. 1a, 1 b and 1 c after the primary structures have been filled witha flowable material and solidified.

FIGS. 3a, 3 b and 3 c show cross sections of the structures of FIGS. of2 a, 2 b and 2 c after the flowable material has been removed down tothe face of the primary structure.

FIGS. 4a, 4 b and 4 c show structures obtained after the structures ofFIGS. 3a, 3 b and 3 c have been covered with a conductive layer orlaminate having a conductive layer.

FIGS. 5a, 5 b and 5 c show secondary structures which are obtained byseparating the primary structure of FIGS. 2a, 2 b and 2 c from thesolidified flowable material.

FIGS. 6a, 6 b and 6 c show microstructures obtained by removing thesolidified flowable material from the microstructures of FIGS. 5a, 5 band 5 c to form microstructures having through-apertures.

FIGS. 7a, 7 b and 7 c show secondary structures in cross sectionobtained by lifting or dissolving the primary structure shown in FIGS.4a, 4 b and 4 c from the conductive layers.

FIGS. 8a, 8 b and 8 c show structures in cross section obtained bycovering the structures of FIGS. 7a, 7 b and 7 c with a metal layer.

FIGS. 9a, 9 b and 9 c show metal microstructures obtained by removingthe conductive layers shown in FIGS. 8a, 8 b and 8 c.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention allows one to prepare a variety ofmicrostructures which have the same structure as an initialmicrostructured mold (the primary structure) or which are complementaryto this initial mold. The mechanical machining processes as well as theadditive structuring and subtractive structuring processes are conductedby processes well known in the art. Similarly, the filling of theprimary structure with a flowable material is conducted usingconventional filling processes such as reaction-injection moldingprocesses. The separation of complementary microstructure elements isconducted by physically separating the elements (lifting-off) or may beconducted using conventional dissolution methods.

The primary structure is produced from metal (for example brass,aluminum or steel), ceramic (for example aluminum oxide, porcelain orhard metal), glass (for example borosilicate glass, calcium fluorideglass, lithium fluoride glass or lithium niobate glass), stone (forexample precious stones such as sapphire, ruby or topaz), plastics (forexample thermoplastics, reaction-curable resins) or a combination ofthese materials (for example a laminate consisting of a metallic supportand a plastic layer)—preferably from silicon, brass, quartz, galliumarsenide, germanium, polysulfone or polymethylmethacrylate—by precisionmechanical machining, such as sawing, grinding, milling or drilling, ifappropriate using diamond tools, laser machining, diamond machining orother precision methods. The solid body can be structured further byadditive structuring, i.e. by imagewise application ofmaterial—preferably by physical or chemical deposition from the vaporphase (PVD or CVD). Monocrystalline material—such as silicon, quartz orgermanium—can be structured by subtractive structuring, i.e. byimagewise removal of material, preferably by anisotropic etching or ionetching. Depending on the properties of the solid body on which theprimary structure is to be produced, two of these three methods or allthree methods can be combined with one another.

Suitable flowable materials for filling and covering the primarystructure are reaction-curable resins, which are solidified by curing,or molten plastics, which are solidified by cooling. It is also possibleto use a pulverulent material—preferably a metal, ceramic, glass orplastic powder—or a slip composition containing one of these powders.The pulverulent material or the slip composition is solidified bydrying, sintering or firing.

The solidified flowable material is separated from the primary structureby lifting off the primary structure or by selective dissolution of theprimary structure. This gives a metal, plastic or sintered materialmicrostructure element whose structure is complementary to the primarystructure.

Precision mechanical removal of the layer covering the primary structuregives a metal, plastic or sintered material microstructure element withthrough-apertures (holes). Depending on the structural shape, thecovering layer can be removed from the primary structure before or afterthe separation of the solidified flowable material. To produce a metalmicrostructure element whose structure corresponds to the primarystructure, the layer covering the primary structure is removed, afterthe flowable, non-electroconductive material has solidified, as far asthe front face of the primary structure by precision mechanicalmachining. A top layer is then applied which is in contact with thefront face of the primary structure. This top layer comprises aconductive material, for example conductive plastic or metal, or alaminate; the front face of the primary structure is covered by a thinmetal layer, on top of which is a thick plastic layer. The solidifiedflowable material, together with the top layer, is then separated fromthe primary structure—preferably by lifting—giving the secondarystructure, which is complementary to the primary structure. Thissecondary structure is filled or covered by an electrodeposited orchemically deposited metal such as nickel, copper or gold. Themicrostructured layer is separated from the secondarystructure—preferably by lifting—giving the metallic microstructureelement, whose structure corresponds to the primary structure, butcomprises a different metal than the primary structure.

This metallic microstructure element can be used as such or used (as atertiary structure) for casting microstructure elements whose structureis complementary to the primary structure.

For producing the primary structure, a material is selected whoseproperties satisfy the demands of the stated structuring process asfully as possible. These include

very low-burr structurability during precision mechanical machining,

mirror-smooth surfaces of the resultant microstructures,

very high material homogeneity and purity,

adequate dimensional stability aid mechanical strength for thestructuring and subsequent process steps, and

selective etchability with respect to the other substances presentduring etching.

Particularly suitable are comparatively thick sheets of amonocrystalline material—preferably made from silicon, quartz, galliumarsenide or germanium—which are employed in large amounts inmicroelectronics, are readily available and are inexpensive. Inaddition, sheets or, if desired, other elements (such as cylinders) madeof glass, ceramics, stone or other metals are also suitable if theysatisfy the requirements of the intended structuring method.

For stability reasons, preference is given to sheets with a thickness ofabout 2 mm or more which are structured on one side in theabove-mentioned manner. If the primary structure is later to beseparated from the secondary structure or the microstructure element bylifting, the primary structure must be free from undercuts, narrowingcavities, burrs and rough side surfaces, since this makes lifting offmore difficult or impossible. If the primary structure containsundercuts or narrowings, the primary structure is dissolved and thusseparated from the secondary structure.

The primary structure is preferably filled and covered by means of athermoplastic or a reaction-curable resin. Suitable thermoplasticsinclude C₁₋₈ alkyl(meth)acrylate resins. Particularly suitable ispolymethyl methacrylate, due to its good flow properties and itsadequate chemical and mechanical stability for the subsequent processsteps. Further suitable thermoplastics are for example polysulfone,polyethylene and polypropylene. Even reaction-curable resins for examplebased on polyamide, polyimide, polypyrrole or polymethylmethacrylate aresuitable.

Pulverulent flowable material or a slip composition is poured ormechanically pressed into the primary structure, if necessary withapplication of a vaccum. Sinterable pulverulent materials are forexample metal powders (such as ferrous alloys, alloys of tin and zinc orcopper-containing alloys), ceramic powders (such as zirconium oxide,aluminum oxide or zirconium oxide strengthened cordierite) or glasspowders (such as sodium-calcium-silicate or a glass powder produced bythe sol-gel-process).

The layer covering the primary structure enables or simplifies theseparation operation and may be a constituent of the microstructureelement containing cavities open on one side. In one embodiment,mechanical removal of the top layer gives microstructure elements withthrough-apertures, such as sieve and net structures. In anotherembodiment, the layer covering the primary structure is removedmechanically, and a layer of electroconductive material is applied ifthe microstructure made from solidified flowable material issubsequently to be recopied, for example by electroforming, to give ametallic microstructure. For the electroforming of microstructureshaving a large aspect ratio, cavities whose side walls arenon-conductive are generally required, so that the electrodepositedmetal only grows up from the conductive base layer. Other structuralshapes such as pyramid structures—are also completely filled byelectroforming in the case of conductive side walls.

The conductive layer comprises, for example, a conductive plastic,adhesive or metal. Conductive thermoplastics may contain a filler whichis conductive; these are converted, for example, to sheets which arewelded or adhesively bonded to the solidified flowable material fillingthe cavities of the primary structure. Plastics having intrinsicconductivity, for example polypyrrole, polyacetylene and polythiophene,can likewise by used as the conductive layer.

A conductive layer of metal is produced, for example, byvapor-deposition with metal at relatively low temperatures.Electroforming only requires a very thin metal layer, which is easy toproduce by vapor deposition.

In order to improve the mechanical stability, the thin metal layer isadhesively bonded, for example, to a plastic plate or reinforced byelectrodeposition of metal.

The conductive top layer must have direct contact with the front facesof the primary structure to give cavities having a conductive base layerafter the separation operation.

In order to separate the primary structure from the solidified flowablematerial, it is possible in the case of many structures to lift thesolidified flowable material off the primary structure, in particular ifthe solidified flowable metal has sufficient inherent stability, onlyadheres weakly to the primary structure and has no undercuts ornarrowings. In separation of this type, the primary structure isretained and can be refilled and covered with flowable material, i.e. itcan be used repeatedly for the production of microstructure elements orfor the production of the secondary structure.

On the other hand, the primary structure can be separated from thesolidified flowable material by chemical dissolution of the primarystructure, which is then lost. Primary structures of crystallinematerial are attacked by basic etchants or salts to which many metalsand plastics are resistant.

Sodium hydroxide solution and potassium hydroxide solution are suitableetchants for silicon. Sodium hydroxide solution containing hydrogenperoxide is a suitable etchant for gallium arsenide. Ammonium bifluorideis a suitable etchant for quartz, and a mixture of hydrogen peroxide andphosphates is a suitable etchant for germanium. For complete dissolutionof silicon, an 18% strength potassium hydroxide solution at 70° C., forexample, is suitable. Polymethyl methacrylate and metals are resistantto this solvent.

A secondary structure having a conductive base layer or base plate canbe copied again by electroforming, giving a metallic microstructureelement whose structure corresponds to the primary structure, but whichcomprises a different metal than the primary structure. This element maybe the desired end product. On the other hand, this microstructure(tertiary structure) can be used as a mold insert for repeated castingof a microstructure element whose structure is complementary to theprimary structure. Some of the process steps indicated can beinterchanged, allowing the process sequence to be varied.

The process according to the invention and the microstructure elementsproduced by the process have the following advantages:

(1) In the simple case, the production of a microstructure elementrequires only two essential process steps, namely structuring of thesolid body by precision machining, additive structuring or subtractivestructuring, and the filling and covering of the primary structure witha flowable material.

(2) The process is carried out without deep X-ray lithography; extended,expensive irradiation times are thus unnecessary.

(3) For the production of the primary, secondary or tertiary structure,materials are selected which are ideal for these process steps;requirements of the microstructure elements are not taken into accounthere.

(4) For the microstructure elements, materials are selected whichideally satisfy the requirements of these elements; requirements madeduring the production of the primary, secondary or tertiary structurehave no further effect.

(5) The microstructures can have different heights from area to area.

(6) In addition to the structural shapes obtainable by deep X-raylithography, other shapes which can be produced rapidly andinexpensively are accessible.

(7) Microstructures having mutually inclined or curved walls can beproduced simply and at little cost.

(8) The volume shrinkage which may occur on the solidification of theflowable material is compensated by the top layer on the primary orsecondary structure.

FIG. 1 shows three examples, each in an angled view, of the primarystructure (microstructure) produced from a solid body. The primarystructure in FIG. 1a is obtained by precision mechanical machining, thatin FIG. 1b by subtractive structuring and that in FIG. 1c by additivestructuring. FIGS. 2a, 2 b and 2 c show a cross-section through theprimary structure (1) which has been covered and filled with a flowablematerial (2) and can subsequently be machined further in various ways.On the one hand, the top layer can be removed as far as the front faceof the primary structure, giving the primary structures (1) filled withflowable material (2) shown in FIGS. 3a, 3 b and 3 c. Covering with aconductive layer gives the structures shown in FIGS. 4a, 4 b and 4 c.FIG. 4a shows a relatively thick top layer (3) of conductive plastic,FIG. 4b shows a laminate comprising a thin metal layer (4) directly onthe front face of the primary structure and a relatively thick layer (5)of non-conductive plastic on top of the metal layer, and FIG. 4c shows arelatively thick metal layer (6).

The primary structure is then separated from the secondary structure, inthe case of the structures in FIGS. 4 a and 4 c, for example, bylifting, in the case of the structure in FIG. 4b, for example, bydissolution of the primary structure. This gives the secondarystructures shown in FIGS. 7a, 7 b and 7 c, having conductive layers (3),(4) or (6) on the front face of the primary structure, and thestructured solidified flowable material (2), which has filled thecavities of the primary structure. In FIG. 7b, the thin conductive layer(4) is reinforced by a relatively thick layer (5).

If the primary structure can be lifted off the secondary structure, theprimary structure can be used repeatedly for the production of thesecondary structure.

The cavities (7) of the secondary structures in FIGS. 7a, 7 b and 7 care filled and covered with metal (8), for example by electroforming;FIGS. 8a, 8 b and 8 c show the layer sequence.

The metal microstructure is separated from the secondary structure, togive the metal microstructure elements shown in FIGS. 9a, 9 b and 9 c.The structures shown in FIGS. 8a and 8 c can be separated bylifting-off. In the case of the structure in FIG. 8b, the two top layers(4) and (5) and the solidified flowable material (2) originating fromthe primary structure are dissolved.

The metallic microstructure elements according to the invention shown inFIGS. 9a, 9 b and 9 c are used directly or serve as a mold insert forcasting of complementary microstructure elements, for example in FIGS.9a and 9 c.

On the other hand, the structures shown in FIGS. 2a, 2 b and 2 c can beseparated from one another, giving the structures of solidified flowablematerial (2) shown in FIGS. 5a, 5 b and 5 c. In order to separate thecomposite element shown in FIG. 2b, the solid body (1) forming theprimary structure is dissolved.

The plastic or sintered material microstructures shown in FIGS. 5a, 5 band 5 c and the metal or sintered material microstructures shown inFIGS. 9a, 9 b and 9 c have cavities (9) open on one side and aremicrostructure elements according to the invention.

Furthermore, the top layer of solidified flowable material can beremoved mechanically from the microstructures in FIGS. 5a, 5 b and 5 c,giving the microstructure elements according to the invention shown inFIGS. 6a, 6 b and 6 c, which, in the example shown here, compriseplastic or sintered material, with through-apertures (10).

Other features of this invention will become apparent in the course ofthe following description of an exemplary embodiment which is given forillustration of the invention and is not intended to be limitingthereof.

EXAMPLE

A nickel microstructure element with cavities open on one side isproduced, for example, as follows.

A polished sheet of silicon (thickness 2 mm, diameter 100 mm) isstructured by sawing using a 70 μm thick diamond saw blade. Themicrostructure comprises square columns; (140 μm wide in each direction,600 μm high). The circular sheet is subsequently divided into aplurality of rectangular pieces by means of a coarser saw blade.

The silicon sheet containing the primary structure is cleaned andadhesively bonded to a metal support, which is installed in the mold ofa reaction-injection molding machine. The primary structure iscompletely filled with polymethyl methacrylate plastic with applicationof a vacuum and covered with an approximately 2 mm thick layer; theplastic cures completely within a few minutes.

The metal support containing the bonded-on and filled primary structureis removed from the molding machine. The layer covering the primarystructure is removed by milling, exposing the front face of the siliconsheet.

A conductive layer of titanium and titanium dioxide is vapor-depositedon the front face of the structured silicon sheet and provided with acontact wire. A support plate of polymethyl methacrylate is adhesivelybonded to the conductive layer. The primary structure is separated fromthe secondary structure by lifting-off.

Nickel is electrodeposited in the cavities of the secondarystructure—starting from the conductive layer on the base of each cavity.The nickel-filled cavities are covered with a nickel layer with athickness of several millimeters as a shape-stable support layer. Thesecondary structure is separated from the nickel microstructure elementby lifting-off. This secondary structure is used to produce furthermicrostructure elements from plastic.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A process for the production of amicrostructured element, comprising the steps of: (a) forming amicrostructure of a microstructured mold having an open cavity on onesurface thereof from a solid body by mechanical machining, additivestructuring or subtractive structuring, wherein said mold consistsessentially of ceramic, glass, stone, quartz, gallium arsenide,germanium or a mixture thereof; (b) filling the cavity and covering themicrostructured mold with a flowable plastic or sinterable material; (c)solidifying the flowable material which has filled and covered themicrostructured mold; and (d) separating the solidified flowablematerial form the mold to provide a plastic or sintered material elementhaving a microstructure complementary to the microstructured mold. 2.The process of claim 1, wherein said mold comprises metal, ceramic,glass or stone.
 3. The process of claim 1, wherein said mold comprisessilicon, quartz, gallium arsenide or germanium.
 4. The process of claim3, wherein said mold comprises quartz or germanium and said mold isformed by anisotropic etching or ion etching.
 5. The process of claim 1,wherein said mold is prepared by additive structuring, wherein saidadditive structuring is physical or vapor deposition onto said solidbody.
 6. The process of claim 1, wherein said flowable material is areaction-curable resin and said solidifying occurs by curing saidreaction-curable resin.
 7. The process of claim 1, wherein said flowablematerial is a molten plastic and said solidifying occurs by cooling. 8.The process of claim 1, wherein said flowable material is ceramic orglass powder or a slip composition thereof and said solidifying occursby drying, sintering or firing.
 9. The process of claim 1, wherein saidseparating step comprises physically lifting said solidified flowablematerial from said mold.
 10. The process of claim 1, wherein saidseparating step comprises selective dissolution of said mold.
 11. Theprocess of claim 1, further comprising: (e) removing a portion of saidplastic or sintered material from said microstructured element having amicrostructure complementary to said mold to produce a secondmicrostructured element having apertures extending through said secondelement.
 12. The process of claim 11, wherein said second element doesnot have protrusions, undercuts or cavities.
 13. The process of claim 1,further comprising: (e) removing a portion of said plastic or sinteredmaterial from said element having a structure complementary to said moldto produce a second element having protrusions, cavities, undercuts orcombination thereof, but not holes or perforations in said secondelement.
 14. A microstructural element produced by a process comprisingthe steps of: (a) forming a microstructure of a microstructured moldhaving an open cavity on one surface thereof from a solid body bymechanical machining, additive structuring or subtractive structuring,wherein said mold consists essentially of ceramic, glass, stone, quartz,gallium arsenide, germanium or a mixture thereof; (b) filling the cavityand covering the microstructured mold with a flowable electricallynon-conductive material; (c) solidifying the flowable electricallynon-conductive material to form a solid layer in contact with thesurface of the microstructured mold and filling the cavity; (d) removinga portion of the solid layer to expose the surface of themicrostructured mold and the cavity filling with solidified flowablematerial; (e) applying a layer of conductive material to and coveringthe exposed surface of the mold and filled cavity; (f) separating theconductive layer and solidified flowable material in the cavity from themold to provide a microstructure having a shape complementary to themold; (g) electrodepositing a metal layer on the complementarymicrostructure; and (h) separating the metal layer from thecomplementary structure to provide a metallic microstructured element.